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arXiv:2604.23444 · npj Quantum Information

Fiber-integrated Quantum Frequency Conversion for Long-distance Quantum Networking

Zhichuan Liao, Ao Shen, Lai Zhou, Nan Jiang, Zhiliang Yuan

quant-phSubmitted Apr 25, 2026• Announced Apr 28, 2026• 0 citations
arXivHTMLPDF

Abstract

Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.

Journal reference
npj Quantum Information, 2026, published online (unedited version)
DOI
10.1038/s41534-026-01225-y
Comments
10 pages, 5 figures, 3 tables

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Signal photons emitted by quantum nodes typically fall outside the low-loss telecom window of optical fibers, leading to severe transmission losses. Quantum frequency conversion (QFC) offers an effective optical interface that bridges quantum nodes with telecom-band channels, enabling long-distance quantum communication. In this work, we demonstrate a compact, fiber-integrated QFC system with low noise and a high signal-to-noise ratio (SNR). Using a periodically poled lithium niobate (PPLN) waveguide, input photons at 637.2 nm are down-converted to telecom photons at 1588.3 nm. Our system achieves a total conversion efficiency of approximately 9%, with pump-induced noise suppressed to 154 Hz. For input photon rates of 32.7, 118.0, and 327.7 kHz, the corresponding SNRs are 12.3, 43.9, and 117.8, respectively. We further develop a theoretical model to simulate the entanglement fidelity between nitrogen-vacancy (NV) center spins and the frequency-converted telecom photons. At the emission rate of an NV center, our QFC system maintains an expected fidelity exceeding 52% over a transmission distance of 100 km. These findings highlight the potential of our QFC system for scalable, long-distance quantum networking.
Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. In this work, we demonstrate a short-wavelength-pumping QFC with the first-order quasi-phase matching period of 3.07 um on thin-film lithium niobate, converting ultraviolet photons to the telecom C-band. By constructing a theoretical model that correlates the normalized conversion efficiency with domain defects in the short-period phase-matched waveguide, we found the critical tolerance of domain defects along the waveguide should be ≤2\le 2≤2 (excluding the ends). Based on this, we achieved a theoretical limit normalized conversion efficiency of 839%/(W*cm^2) for the fundamental guided mode through fabrication optimization. Furthermore, we propose a robust noise suppression strategy for short-wavelength pumping by utilizing the counter-tuning behaviors of difference-frequency generation and spontaneous parametric down-conversion. By combining these advances with ultra-narrowband filtering, we achieve a record-high external efficiency of 28.8% and an ultra-low noise of 35 counts per second. This high-performance QFC connecting ultraviolet and telecom bands satisfies the stringent requirements for long-lived remote ion-ion entanglement in scalable quantum networks [W.-Z. Liu et al., Nature (2026)].
High-efficiency quantum frequency conversion (QFC) for group-IV color centers, such as silicon vacancies (SnV), is required for long-distance quantum networking protocols. SiV QFC has previously been demonstrated at external eGiciencies of 12% [1] and 30% [2], using periodically poled lithium niobate (PPLN) waveguides of lengths 3.5 and 5 mm, and with eGiciencies limited by the available pump power (120 mW and 320 mW, respectively). Here, the eGiciency scales with the square of the waveguide length. These results are also obtained with pump wavelength of 1623 nm (L-band), networking the SiV wavelength of 737 nm into the O-band (1350 nm). Here, we improve these results by replacing the pump (L-band in [1],[2]) with a pump with wavelength in the C-band (1561 nm), where (i) high-power erbium-doped fiber amplifiers (EDFAs) are available (ii) SMF-28e fibers have lower losses in the E-band than O-band. Using an EDFA and a 1.5-cm-long PPLN waveguide We measure a maximum external photon conversion eGiciency of 43±1.8% (corresponding internal eGiciency of 96±1.8%), which we reach at an external pump power of 1.51 W. The external eGiciency is limited by waveguide-fiber coupling. As such, we have demonstrated an external eGiciency for SiV-telecom quantum frequency conversion exceeding prior results from MIT, Lincoln Labs, and Harvard. To further improve these results in the future, the waveguide will be replaced with a longer one to reduce the power required to reach unit internal eGiciency, which further reduces noise. We may also optimize fiber coupling or switch to a free-space-coupled device, which we estimate to increase the external conversion eGiciency to approx. 70-80%.
In a fiber-based quantum network, the utilization of the telecom band is crucial for long-distance quantum information (QI) transmission between quantum nodes. However, the near-infrared wavelength is identified as optimal for storing and processing QI through alkaline atoms. Recognizing the challenge of efficiently bridging the frequency gap between atomic quantum devices and telecom fibers while maintaining the QI carried by photons, quantum frequency conversion (QFC) serves as a pivotal quantum interface. In this study, we explore an efficient telecom-band QFC mechanism based on diamond-type four-wave mixing (FWM) with rubidium energy levels. The mechanism enables the conversion of photons between the near-infrared wavelength of 795 nm and the telecom band of 1367 or 1529 nm. Using the Heisenberg-Langevin approach, we optimize conversion efficiency (CE) across varying optical depths while considering quantum noises and present corresponding experimental parameters. Unlike previous works neglecting the applied field absorption loss, our results are more relevant to practical scenarios. Moreover, by employing the reduced-density-operator theory to construct a theoretical framework, we demonstrate that this diamond-type FWM scheme can maintain the quantum characteristics of input photons with high fidelity, such as quadrature variances and photon statistics. Importantly, these properties remain unaffected by vacuum field noise, enabling the system to achieve high-purity QFC. Another significant contribution lies in examining how this scheme impacts QI encoded in photon-number, path, and polarization degrees of freedom. These encoded qubits exhibit remarkable entanglement retention under sufficiently high CE. In the case of perfect CE, the scheme can achieve unity fidelity. This comprehensive exploration establishes a theoretical foundation for the application of the diamond-type QFC scheme based on atomic ensembles in quantum networks, laying essential groundwork for advancing the scheme in distributed quantum computing and long-distance quantum communication.
Quantum frequency conversion (QFC) is essential for bridging the spectral gap between stationary qubits and low-loss optical communication channels. Practicable QFC interfaces connecting ultraviolet and telecommunication bands are of great significance but have remained elusive, constrained by the dual challenges of a high-performance frequency converter and effective noise suppression in short-wavelength pumping configurations. We address this by demonstrating the phase-matching mechanism with domain defects and differential-tuning behavior of the spontaneous parametric down-conversion noises. Based on a quantitative analysis of performance degradation induced by domain defects, we propose specific fabrication guidelines and experimentally achieve an ultrahigh normalized conversion efficiency of 839%/(W cm^{2}) for the fundamental guided mode in thin-film lithium niobate waveguides. Meanwhile, we reveal a new noise control mechanism for short-wavelength pumping by utilizing the differential-tuning behaviors of difference-frequency generation and spontaneous parametric down-conversion processes. Then a single-stage QFC from 393 nm to the telecom C band with a record-high external efficiency of 28.8% and an ultralow noise level of 35 counts per second is achieved experimentally. The proposed mechanisms would enable other QFCs with higher efficiency and lower noise, advancing the development of scalable quantum networks. Furthermore, it holds significant potential for driving progress in both classical and integrated nonlinear photonics.
Quantum frequency conversion (QFC), which involves the exchange of frequency modes of photons, is a prerequisite for quantum interconnects among various quantum systems, primarily those based on telecom photonic network infrastructures. Compact and fiber-closed QFC modules are in high demand for such applications. In this paper, we report such a QFC module based on a fiber-coupled 4-port frequency converter with a periodically poled lithium niobate (PPLN) waveguide. The demonstrated QFC shifted the wavelength of a single photon from 780 to 1541 nm. The single photon was prepared via spontaneous parametric down-conversion (SPDC) with heralding photon detection, for which the cross-correlation function was 40.45 ± 0.09. The observed cross-correlation function of the photon pairs had a nonclassical value of 13.7 ± 0.4 after QFC at the maximum device efficiency of 0.73, which preserved the quantum statistical property. Such an efficient QFC module is useful for interfacing atomic systems and fiber-optic communication.
For quantum information processing (QIP), it is important to have a long-lived quantum memory (QM), coupled to other QMs and quantum processors (QP). However, QM and QP systems demonstrated so far suffer from many limitations, and in the near future a single platform will not have the optimal version of all the components needed for QIP. Thus, it is also important to be able to couple quantum bits in different systems, for example, Rb atoms and NV diamond, preferably using telecom fibres. In this paper, we describe a quantum frequency converter (QFC) that will perform this telecom band qubit conversion. The QFC is based on periodically poled lithium niobate waveguides. For concreteness, we consider specific examples: the conversion of a 780 nm or 795 nm Rb qubit to the telecom band and the conversion of a 637 nm photonic NV diamond qubit to the telecom band, as well as the inverse processes. We show that interface fidelity exceeding 95% should be feasible. Given the storage times ∼1 s already demonstrated in the solid-state systems, and the recent demonstrations of spin-photon entanglement with the NV and entanglement of the NV spin with a superconducting flux qubit operating in the microwave region near 2.88 GHz, such a link would provide the key interface needed to build a quantum internet.